The present invention relates to container closures, and in particular to closures for use with containers which are hot filled, sealed, and exposed to moisture.
Many containers or packages, for example those filled with certain foods, are filled at elevated temperatures, or xe2x80x9chot filled.xe2x80x9d In the hot filling process, the product is to introduced to the container at a temperature which can kill bacteria in the product as well as bacteria on the interior surface of the container. In this manner, the food inside the container is sterilized without any pretreatment of the container, increasing manufacturing efficiency. After hot filling, containers may be capped and passed through a cooling bath to drop the temperature of the product. The cooling bath can be, for example, a submersion tank filled with water or a cooling tunnel housing a water spray.
Glass containers and metal closures have been employed in the hot filling of a variety of products. When these are used, the interior top surface of the metal closure may be coated with a closure lining compound such as plastisol. The process of cooling the heated product tends to reduce the pressure inside the container, creating a partial vacuum effect. This vacuum retains the metal closure tightly against the glass container neck, and the lining compound then effectively seals the contents of the container from spoilage. The lining compound also prevents water from migrating into the container, which may be drawn into the container from the cooling bath due to the vacuum created as the container and contents are cooled. While glass containers are effective, however, they are much more expensive than equivalent plastic containers, and are of course relatively fragile. Similarly, metal closures are relatively expensive and can rust or otherwise degrade.
Accordingly, it is desirable to employ plastic containers and closures that may be hot-filled. However, when plastic containers and closures are employed, it is difficult to utilize a closure lining compound, because curing these compounds typically requires exposure to temperatures which would melt or otherwise degrade the plastics. As a result, a different method of sealing the product must be used. One alternative sealing mechanism is a liner, such as a foil liner, which is sealed to the face of a container neck. Sealing liners are especially advantageous when contents may be stored for long terms, for example while food products are shipped, stored, and displayed on shelves.
As noted above, the process of filling at elevated temperatures, sealing the container, and cooling the container tends to draw water under the closure top. Unlike the liner compound of a metal closure, however, which is bonded to the top wall of the closure, a foil liner is generally bonded to the container neck. As a result, the area between the foil liner and the top wall of the closure may be accessible to moisture. This arrangement may promote water migration between the inner surface of the top wall and the outer surface of the liner. The presence of water in this area is undesirable due to potential growth of bacteria and molds, which may occur over time as the product is shipped and stored. This growth may be particularly significant given that the cooling bath water is normally not purified.
In order to prevent water migration, some known closure systems provide a secondary liner to obstruct water, for example a liner of polyolefin foam or a compression molded gasket made from thermoplastic rubber. These types of closures rely on compression of the secondary system to prevent water ingress. The addition and compression of the secondary liner, however, require additional closure elements and additional manufacturing steps, adding to the cost of the closure. Moreover, to the extent that these materials are porous or absorbent, bacterial growth may nevertheless occur. The prior art has therefore not provided a plastic closure for use with hot-filled plastic containers that effectively prevents water migration and bacterial growth between a container liner and the inside surface of the closure.
The present invention provides a closure system for a plastic container, which includes a liner and a plastic closure. The liner can be, for example, induction sealed to the neck of a container at the end face of the neck and, for example, along an outer surface of the neck. The plastic closure may engage the liner to retain the liner against the neck during induction sealing and to seal the area between the liner and the closure top against migration of liquids. The closure preferably includes a closure top wall, an annular closure skirt depending from the top wall, an annular outer flexible seal depending from the top wall, and at least one annular pressure ring. The outer flexible seal can optionally retain an outer edge of the liner against the outer surface of the container neck and minimize ingress of liquids. The annular pressure ring may engage the liner to retain the liner against the end face of the neck during induction sealing and prior to removal of the closure.